A large number of catalysts for producing methacrylic acid by subjecting methacrolein, isobutylaldehyde or isobutyric acid to gas phase catalytic oxidation, have been proposed. Most of the catalysts thereof are mainly composed of molybdenum and phosphorus, and have structures of heteropolyacids and/or salts thereof. The catalysts used in the reaction, however, have low reaction activities, low selectivity for the desired substance, and short lives, as compared to molybdenum-vanadium based catalysts used in reactions for producing acrylic acid by subjecting acrolein to gas phase catalytic oxidation, which are known as reactions similar to the reaction for producing methacrylic acid. Accordingly, the improvement of catalytic performance of the catalysts is required although some of the catalysts are industrially utilized.
The present inventors first tried the improvement of low activities, low selectivity and short lives of conventional gas phase catalytic oxidation catalysts for methacrolein, and found out that gas phase catalytic oxidation catalysts for methacrolein prepared by the addition of a variety of elements to Mo, V and P, have heteropolyacid (salt) structures and have high activities, high selectivity and are particularly stable for the time lapse. The inventors propose the catalysts described in Japanese Patent Publication No. 58-11416, Japanese Patent Publication No. 59-24140, Japanese Patent Publication No. 62-14535 and Japanese Patent Publication No. 62-30177.
Recently, because of high concentrations of raw material gases and of environments under which oxidation reactions are conducted at elevated temperature, catalysts that exhibit further high activities, high selectivity and long lives, are needed. Various preparation methods are proposed to provide catalysts that satisfy these demands. For example, Japanese Patent Laid-Open No. 5-31368 and Japanese Patent Laid-Open No. 8-196908 propose methods for preparing molding catalysts that involve using NH4 in addition to the components of Mo, V and P, and utilizing aqueous ammonia as the ammonium source. In addition, Japanese Patent Laid-Open No. 11-226411 describes a method for preparing a molding catalyst that comprises using refined starch when an active component of the catalyst is granulated, and improving the pore volume of the catalyst by burning the starch in the calcining step.
Furthermore, when a catalyst is loaded in a fixed-bed reactor as an industrial catalyst, the catalyst is required to be molded to a constant size in order to reduce the pressure drop of the reaction gas prior to and subsequent to the catalyst layer. For this purpose, known methods involve normally molding a catalyst powder to a cylindrical material, a pellet, a ring-shaped material, a sphere-shaped material, or the like, and impregnating or coating an inert carrier with an active catalyst material also.
Advantages of a coated catalyst having the inert carrier as the core include [1] being capable of improving the effective utilization factor of active components of the catalyst, [2] being expected to improve the selectivity due to the homogeneous distribution of the residence time of reaction materials within the catalyst, and [3] facilitating the removal of the reaction heat on account of the improvement of the catalyst thermal conductivity or the dilution effect of the inert carrier. As a result, there are many examples applied to selective oxidation of a large heat release.
On the other hand, technical disadvantages in preparing a coated catalyst include [1] the peeling of the coating layer and the difficulty of obtaining a mechanically strong catalyst because the catalyst is subject to cracking, [2] the difficulty of coating a carrier with a large amount of active catalytic material, and [3] the difficulty of obtaining a highly active catalyst due to inclusion of inert materials.
Methods for overcoming the disadvantages are related to the properties of active catalyst substances and the present situation is to study catalysts individually because of no general techniques.